EXTERNAL ANATOMY. CHESTS. 31 
Brazilian forests, which have scared us from a steady 
aim of our gun, by the sudden manner in which they 
threw up their crests the moment they discovered their 
danger, uttering at the same time a loud and discordant 
scream. The sensation, it is true, lasts hut for a mo- 
ment ; but the whole is so sudden and unexpected, that 
the sportsman is involuntarily startled ; and this mo- 
mentary feeling gives time for the bird to dart among 
the thick foliage of the forest, and thus effect its escape. 
The crest of a bird is always erected under a sense of 
danger or of anger, as every body knows who has seen 
a cockatoo ; so that it has obviously been intended by 
nature to perform the office of intimidating, however 
momentarily, the foes of its possessor. We have else- 
where shown* that crests are one of the most certain 
indications of rasorial types ; because this appendage is 
intended to be analogous to the horns of ruminating 
quadrupeds, which the rasorial birds represent : and 
we shall now explain the various forms they assume. 
(40.) All the crests of birds, however diversified, 
may be referred to three primary forms, or types. The 
first is mobile, and reposes in a flat or horizontal posi- 
tion on the back of the head, an<l is elevated and ex- 
panded when displayed. The second is almost mobile, 
but the feathers are not lengthened ; it opens, as it were, 
from the midst of the crown, and radiates from a com- 
mon centre ; this we shall call a concealed crest ; for in 
a state of repose, from the feathers not being elongated, 
it can only be discovered by raising them up. The 
third sort of crest is fixed — the feathers being, as it were, 
compressed on the sides of the head, so that their ends 
form an elevated keel, like the ridge of a helmet ; and 
this always remains, more or less, in the same posi- 
tion. 
(41.) Mobile Crests are the most common, and are 
consequently the most varied ; hence we can trace 
among them five different modifications. The first in- 
Introductory Discourse, p. 253. Classification of Animals, p. 264. 
